Drop deploy-root builtin

This one would be better implemented by adding the user-owned repo as
a remote, and just doing pull-deploy.
Drop kvfile

We don't use this anymore for a long time.
Major refactoring

Port upstream JavaScript code to Python.

With this huge commit we:

 * Rewrite the task subsystem and implement chaining #4
 * Drop the use of prefix #5
 * Support building from tarballs #6
Remove event handling

We are about to use GLib for mainloop and signals so we don't need this
anymore.
Remove singleton

It was used for the taskset but we turned it into a module, hence
there is no need for singletons anymore.
Refactor to use glib for main loop and signals

Use GLib main loop and GObject signals instead to roll our own.
We'll need to use GLib for the autobuilder rewrite anyway, because
it will be a DBus service.